Differential signaling is widely employed in modern high-speed communication systems. Its main advantages are lower power consumption, higher noise immunity and lower electromagnetic interference (EMI) as compared with the traditional single ended technologies. Lower voltage swings and larger signal to noise ratios of differential interfaces also allow for higher achievable bandwidths, making them attractive for solutions that require serialization and deserialization of wide parallel busses, resulting in savings both in power and system costs. Differential transceivers can also be easily implemented in complementary metal oxide semiconductor (CMOS) integrated circuit (IC) manufacturing technology considered in this invention.
Traditionally, applications that utilize serial differential interfaces have been found in communication infrastructure, where large amounts of data are transmitted over long distances. Recently, proliferation of portable consumer electronics devices highlighted another potential application of serial differential interface technology that was usually served by single ended solutions. This application is display interface technology. High-resolution displays demand wide parallel interfaces for large number of pixel bits resulting in high costs, power dissipation and system integration difficulties. These problems are especially acute in mobile applications where space, system costs and power dissipation are especially important. In these applications serial differential display interface technology also provides other advantages, such as low EMI emissions and higher robustness against RF interference.
Although serial display interface technology has been known for some time, its use has generally been limited. In one example, separate serializer and deserializer devices convert parallel video data from an application or graphics processor into a serial data stream, which is then converted back into parallel form on the display module. Data is then sent to the display driver in a parallel manner. This architecture primarily addresses system integration problems associated with a large number of physical connections required to transmit high-resolution video data in parallel form. In another example, serial interfaces use different variants of single-ended physical layer technology.
From a system integration perspective a more preferable solution would be a differential serial interface where the serializer and deserializer are integrated within the application/graphics processor and the display driver respectively, thereby increasing power savings, cost efficiency and ease of use. However, while differential serial technology has worked well in communication systems, it has seen little use in display interface applications. At least one reason for this is based upon the properties of materials used for flat panel displays and specifically glass substrate based displays, such as liquid crystal displays (LCDs).
In one approach, the deserializer is integrated into the display driver. Current LCD manufacturing technology typically uses chip-on-glass (COG) based display drivers, which are integrated circuits that are surface-mounted via conductive bumps directly to the surface of the glass, resulting in display module that is compact and suitable for portable applications. In another approach, widely used with larger glass panels for which signals are brought to the edge of the glass, the display driver IC is connected to the glass via a separate add-on printed circuit board (PCB) that is attached to the glass via a special connector.
Referring to FIG. 1, there are two general types of differential signaling technologies: voltage mode and current mode. In both cases a set of switches controls the flow of signaling current from the transmitter through the termination resistor at the receiver and back to the transmitter. In current mode transmitters, signaling current is set with the help of two current sources: one for pull-up path and one for pull-down path. In voltage mode transmitters, signaling current is indirectly set via Ohms law as applied to the regulated supply voltage at the transmitter, driver output impedance and termination resistance RTR.
In either case, the receiver is a voltage mode high-speed comparator that distinguishes the sign of the differential voltage across the termination resistor RTR connected between the input pins DP, DM. The value of termination resistance is critical for correct operation of the interface. It is usually chosen to be equal to the characteristic differential impedance of the transmission lines that connect the receiver and transmitter to eliminate reflection at the receiver inputs. The resistor value also must be high enough to guarantee sufficient differential voltage at the input of the receiver. Hence, the optimal termination resistor value is usually chosen to be in the range of 80-125 ohms for systems using signal transmission media with single ended characteristic impedance Z0 of 50 ohms.
Signal transmission media in typical communication systems use materials with properties that allow construction of transmission lines (TL) for signal propagation. These materials usually have very low direct current (DC) or resistive losses and allow data to be sent over long distances with minimal loss of signal strength. Examples of such transmission media include copper traces on PCB substrates and coaxial cables. In systems on glass, such as LCD displays, transmission media have very different properties characterized primarily by higher DC resistance of the connections thereby making them appear more as discrete resistors rather than transmission lines.
Materials used in COG technology are ITO and ACF. Indium tin oxide (ITO) is a semi-translucent or translucent conducting film used by LCD manufacturers to produce electrical connections on the glass. Mechanical, chemical and thermal properties of ITO make it compatible with many standard IC manufacturing technologies like lithography and etch thereby allowing interconnects to be created with well controlled mechanical and electrical properties. Adhesive conductive film (ACF) is a conductive adhesive that is used to assist in bonding the IC to the ITO traces on the glass, is used as an intermediary step and has insignificant effects on the electrical properties of the chip-to-glass connection after the bonding procedure is completed.
Referring to FIG. 2, an example is shown of a system with an IC bonded to the glass with associated electrical connections on the glass from the IC to the glass edge connector.
Referring to FIG. 3, as noted above, ITO traces on the glass have significant DC resistance RP, e.g., in the range of 50-500 ohms/square, thereby increasing the effective termination resistance of the differential link if the receiver is placed on the glass, as would be the case with the COG display driver. A conventional differential receiver senses the differential voltage at its input pins, and the voltage across these pins will be significantly reduced due to the voltage divider effect produced by the series resistance of the ITO traces.
Referring to FIG. 4, a simple analysis of the equivalent electrical schematic of a conventional interconnect shows that such voltage reduction can be significant enough to reduce the signal-to-noise ratio (SNR) across the inputs DPTR, DMTR of the differential receiver such that performance of the serial interface will be degraded. Further, variability in the properties of ITO materials among manufacturers and under different environmental conditions will result in higher variances in the termination resistance thereby impeding or preventing interoperability between display modules and application/graphics processors from different vendors in different systems.
Referring to FIG. 5, a top level view is provided of a COG system with differential signaling technology to connect a differential transmitter on the PCB to the ICs bonded to the glass. Also depicted are associated parasitic resistances of the ITO traces on the glass. Such variability in termination resistance is incompatible with new industry standard specifications for low power chip-to-chip interconnects for mobile systems, such as those promoted by the Mobile Industry Processor Interface (MIPI) Alliance, where the resistance of the traces on the glass is limited to five ohms. Consequently, a problem to be solved can be summarized as follows: high DC resistances of on-glass interconnects and their manufacturing variability makes standards for on-chip termination of differential signals impractical in COG applications. A solution would address both of these deficiencies without sacrificing advantages of either differential signaling or COG technologies.